1. Field of the Invention
The present invention relates to a ported speaker enclosure of a portable computer.
2. Description of the Related Art
Sealed box or closed cabinet loudspeakers present in conventional portable computers have been limited in loudspeaker excursion by the risk of mechanical and magnetic distortions. Loudspeaker excursion refers to the upward and downward or inward and outward movement of the components of a loudspeaker such as the windings, voice coil, front and back plates, and cone. The risks of mechanical distortions from significant loudspeaker excursion may include banging the windings of the loudspeaker against the front or back plates of the loudspeaker, overstretching the corrugated or folded material known in the art as the “spider” which is typically near the center of the loudspeaker extending between the cone and the frame, or flattening out the curved elastic material known in the art as the “surround” which is typically near the top of the loudspeaker and is positioned on the frame.
A loudspeaker is essentially an electroacoustic transducer that uses a magnetic circuit to achieve acoustic radiation. When current provided to a loudspeaker flows through the voice coil, a magnetic field is produced. As the polarity of the current changes like a sine wave, magnetic waves travel across the gap between the voice coil and the permanent magnet. The gap between the permanent magnet and voice coil of a loudspeaker serves as the primary magnetic flux resistance or reluctance for the magnetic circuit of a loudspeaker. As a result of magnetic waves travelling across the gap, the magnetic field of the voice coil either repels or attracts the magnetic field of the permanent magnet. This causes the loudspeaker to move upward and downward, producing compression and rarefaction of air molecules surrounding the cone of the loudspeaker.
As loudspeaker excursions become significant, components of the loudspeaker become mechanically stiffer and the force constant of these components increases. The increased force constant leads to a higher force reaction, producing mechanical nonlinearities. In addition, the presence of magnetic nonlinearities has been determined by the relationship between the voice coil and the gap. A loudspeaker excursion producing significant magnetic nonlinearities is an excursion that places the voice coil out of the gap. Magnetic nonlinearities have increased as the degree of displacement or deflection of the voice coil out of the gap has increased.
Due to the risk of these mechanical and magnetic distortions, significant loudspeaker excursions associated with acoustic radiation at low frequencies has been avoided in loudspeakers in portable computers. The risk of mechanical and magnetic distortions due to significant loudspeaker excursions is a particularly acute risk in portable computers due to the spatial constraints of a portable computer and the steep low frequency region of a sound pressure level waveform associated with ported speakers. As such, the low frequency acoustic performance of loudspeakers in portable computers is significantly less than the high quality acoustic performance of stand alone loudspeakers at low frequencies.